1. Field of the Invention
The present invention relates generally to a shock absorber having variable damping characteristics. More specifically, the invention relates to a variable damping characteristics valve assembly, which is suitable for use in an automotive suspension system.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 58-81243 discloses a shock absorber laving two way damping characteristics for both piston bounding and rebounding strokes. In order to provide variable damping characteristics for both in piston bounding and rebounding strokes, a first piston has a first bounding and rebounding damping valve, and a second piston has a second bounding and rebounding damping valve. The first and second pistons are arranged in tandem fashion. An intermediate chamber is defined between the first and second pistons. The intermediate chamber is selectively connected to upper and lower fluid chambers of the shock absorber via a switching valve. By switching a fluid connection between the intermediate chamber and one of the upper and lower fluid chambers only one of the first and second bounding and rebounding values in one of the first and second pistons is active for generating damping force.
With the shown construction, since the intermediate chamber is common for both piston bounding and rebounding strokes, it is not possible to control damping characteristics for the piston bounding stroke and the piston rebounding stroke independent of each other. Furthermore, in the shown construction, since first and second pistons are required, the number of parts is unnecessarily great, causing higher cost and requiring complex assembling operation, which causes inefficiency in manufacturing. In addition, due to the presence of the first and second damping valves in the first and second damping pistons, the overall length of the shock absorber becomes excessive.
On the other hand, Japanese Patent First (unexamined) Publication (Tokkai) Showa 58-116213 discloses a variable damping characteristics shock absorber. The shown shock absorber is provided with a damping valve for a piston bounding or compression stroke, which damping valve will be hereafter referred to as the "bounding stroke damping valve" and a damping valve for piston rebounding or expansion stroke, which will be hereafter referred to as "rebounding stroke damping valve. One or more fluid passages are provided in parallel to the aforementioned damping valves. A non-return valve is disposed in the fluid path for permitting fluid flow only in the piston bounding stroke. A by-pass passage is formed by-passing the non-return valve. A variable orifice is provided in the by-passing passage for restricting the fluid flow rate through the by-pass passage. The variable orifice is formed by means of a rotary valve. The rotary valve is rigidly coupled with a control rod which can be rotatingly driven for adjusting the damping characteristics.
With the construction set forth above, the working fluid in a lower fluid chamber of the shock absorber flows through bounding stroke damping valve, through the variable orifice and through the non-return valve during the piston bounding or compression stroke. In each of the working fluid flow routes set forth above, a restriction for the fluid flow is provided for generating damping force.